You can just use a bag with the coupler on the inside and no tip
What Tips Do You Use To Make Balloons On Cake?
You can buy plastic, and cut it in to your own shape balloon, or there are a ton of birthday candles that you can use shaped like balloons!
Try getting a ballon cookie cutter for a stensil otherwise make a stensil out of cardboard
Here is a page on birthday cakes including a balloon cake. Www.birthdaycakeideas.info/